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Cellular Communication

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  1. Cellular Communication ECE 457 Spring 2005 ECE 457

  2. Cellular Phone System • The cellular phone service area is divided into smaller geographical areas called cells. ECE 457

  3. Cellular phone system • Each cell has a base station with a tower which receives and transmits signals. • All the base stations are connected by phone lines to mobile telephone switching office (MTSO). • How does it work? • A caller communicates via radio channel to its base station, which sends the signal to MTSO. • If the called number is land based, MTSO sends the signal through central telephone office like any other phone call. • If the called number is mobile, MTSO sends the signal to the base station of the cell where the called number is. The base station transmits the signal to the called number using the available radio channel. • As the caller moves from one cell to another, MTSO automatically switches the user to an available channel in the new cell. ECE 457

  4. Cell Phones • Cell phones communicate in the high frequency range: 806-890 MHz and 1850-1990 MHz for the newly allocated ‘PCS’ range. • Cells are spaced 1-2 miles apart. • The concept of cells is the key behind the success of cell phones because by spacing many cells fairly close to each other, the cell phones may broadcast at very low power levels (typically 200mW-1W, depending on system). • Since the cell phones may broadcast at low power levels, they use small transmitters and small batteries. • Reuse frequencies at cells that are not adjacent. ECE 457

  5. Encoding and Multiplexing • With thousands of cellular phone calls going on at any given time, everyone cannot talk on the same channel at once. • Therefore, several different techniques were developed by cell phone manufacturers to split up the available bandwidth into many channels each capable of supporting one conversation. • Analog cellular systems use a 3 kHz audio signal to frequency modulate a carrier with transmission bandwidth 30 kHz. ECE 457

  6. FDMA • FDMA (Frequency Division Multiple Access): • It is used on analog cellular systems. • When a FDMA cell phone establishes a call, it reserves the frequency channel for the entire duration of the call. • The voice data is modulated into this channel’s frequency band (using FM) and sent over the airwaves. • At the receiver, the information is recovered using a band-pass filter. • FDMA systems are the least efficient cellular system since each analog channel can only be used by one user at a time. • These channels are larger than necessary given modern digital voice compression and are also wasted whenever there is silence during the cell phone conversation. • Analog signals are also especially susceptible to noise. • Given the nature of the signal, analog cell phones must use higher power (between 1 and 3 watts) to get acceptable call quality. ECE 457

  7. TDMA • TDMA (Time Division Multiple Access): • TDMA builds on FDMA by dividing conversations by frequency and time. • Digital compression allows voice to be sent at well under 10 kilobits per second (equivalent to 10 kHz). • TDMA shares the same channel with multiple sessions. • While TDMA is a good digital system, it is still somewhat inefficient since it has no flexibility for varying digital data rates (high quality voice, low quality voice, pager traffic) . • In other words, once a call is initiated, the channel/timeslot pair belongs to the phone for the duration of the call. • TDMA also requires strict signaling and timeslot synchronization. • Due to the digital signal, TDMA phones need only broadcast at 600 mW. ECE 457

  8. CDMA • CDMA (Code Division Multiple Access): • CDMA uses ‘spread spectrum’ techniques. • CDMA has been likened to a party: When everyone talks at once, no one can be understood, however, if everyone speaks a different language, then they can be understood. • CDMA systems have no channels, but instead encodes each call as a coded sequence across the entire frequency spectrum. • Each conversation is modulated, in the digital domain, with a unique code (called a pseudo-noise code) that makes it distinguishable from the other calls in the frequency spectrum. Using a correlation calculation and the code the call was encoded with, the digital audio signal can be extracted from the other signals being broadcast by other phones on the network.   • Since CDMA offers far greater capacity and variable data rates depending on the audio activity, many more users can be fit into a given frequency spectrum and higher audio quality can be provide. • The current CDMA systems boast at least three times the capacity of TDMA systems. • CDMA technology also allows lower cell phone power levels (200 miliwatts) since the modulation techniques expect to deal with noise and are well suited to weaker signals. • The downside to CDMA is the complexity of deciphering and extracting the received signals. ECE 457

  9. Comparison ECE 457

  10. Spread Spectrum • CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements: 1. The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in immunity to interference and jamming and multi-user access. 2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes. 3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time. • In order to protect the signal, the code used is pseudo-random. This pseudo-random code is also called pseudo-noise (PN). ECE 457

  11. Direct Sequence Spread Spectrum(DS/SS) • CDMA is a DS/SS system. • Signal transmission consists of the following steps: • A pseudo-random code is generated, different for each channel and each successive connection. • The Information data modulates the pseudo-random code (the Information data is “spread”). • The resulting signal modulates a carrier. • The modulated carrier is amplified and broadcast. • Signal reception consists of the following steps: • The carrier is received and amplified. • The received signal is mixed with a local carrier to recover the spread digital signal. • A pseudo-random code is generated, matching the anticipated signal. • The receiver acquires the received code and phase locks its own code to it. • The received signal is correlated with the generated code, extracting the Information data. ECE 457

  12. Spread Spectrum Generation • Pseudo-Noise Spreading • Bit rate of PN is much higher. (chip rate) ECE 457

  13. Spectrum of DS/SS • SS modulation is applied on top of a conventional modulation. • One can demonstrate that all other signals not receiving the SS code will stay as they are, unspread. ECE 457

  14. Properties of DS/SS • Secure Communication • The signal can be detected by authorized persons who know the PN code. • The signal power is small due to spreading (hide signal inside the noise) • Difficult to jam since it is wideband • Multiple Access • Individual users have independent, uncorrelated spreading codes ECE 457

  15. Advantages of CDMA over TDMA and FDMA • Greater capacity • TDMA and FDMA have a fixed number of slots • Frequencies can be reused in all the cells in CDMA. • No hard limit to the number of users. • Resistance to multipath fading. ECE 457

  16. Other Applications of Spread Spectrum • GPS (Global Positioning System) • Determine time, location and velocity of a person • Consists of 24 satellites to measure the exact location • Each satellite uses the same frequency band with DS/SS. • Military Applications ECE 457